The idea that only humans are sentient

Sentience is the capacity to have positive and negative experiences, such as feeling pain and pleasure. Sentience can only exist in a creature who is conscious, because to have an experience means to be conscious of something. Sentience puts the emphasis on the experiences themselves, but in order to experience anything, an individual must be conscious.

Those who deny that nonhuman animals are sentient sometimes claim that it cannot be demonstrated whether animals are sentient. As we don’t really know how a structure must be organized in order to give rise to conscious experience, these critics say we cannot know for sure if the central nervous systems of nonhuman animals are actually those kinds of structures. They also point out that the behavior associated with conscious responses might result from other causes, and suggest that apparently analgesic substances secreted by the bodies of nonhuman animals – which relieve pain, something that would not be needed in a creature who was not conscious of pain – have other purposes. And finally they have argued that the critical step in the evolution of consciousness has taken place only in the last few tens or at most hundreds of thousands of years.

This is often argued by claiming that the development of the neocortex was that critical step. But we have reasons to reject that posit. In fact, we can call this an anthropocentric view. Moreover, it can be characterized as a radically anthropocentric view, since its proponents claim that animals must be assumed not to be conscious because it can’t be proven for sure, despite a great deal of evidence that they are.1 These anthropocentric arguments do not hold up well to scrutiny, for three main reasons.

(1) Animal physiology. It should be noted at the outset that it is impossible to prove today that a being with a centralized nervous system cannot be conscious. So the anthropocentric position suffers at least one of the same problems that skeptics raise, the unprovability of whether or not particular types of centralized nervous systems can give rise to conscious experience.2

(2) Consistency. Those who hold a radically skeptical view about other minds believe that we cannot assume from the available evidence (such as centralized nervous systems and behavioral and physiological indications) the presence or absence of consciousness, and the only acceptable evidence would be if we had access to the mental states of others and could know for sure if they have conscious experiences. If we held a radically skeptical view about other minds, it would be inconsistent to assume that only human beings are conscious. On this view, we should conclude that no individual being other than ourselves is conscious, whether human or nonhuman. That is, we should extend our skepticism to humans, too. The reason is very simple: if we cannot conclude that nonhuman animals are conscious because we have no access to their mental states, then we cannot conclude that human beings are conscious, because we cannot get into their heads and access their experiences, either. This seems, however, highly unlikely, if not practically absurd. Yet this is the view implied by the arguments presented in favor of anthropocentrism, when we take them seriously enough.3

(3) Plausibility and simplicity. We must be guided by the perspective that is most plausible. From the points made above, we see that the radical anthropocentric position cannot prove what it claims to prove, and that if we take its arguments seriously, we reach an implausible conclusion. Since the argument of radical skeptics about the inaccessibility of other minds also applies to other humans, they would also be forced to reject their own assumption that humans other than themselves are conscious. Therefore their hypothesis does not seem to be a plausible one. Most of us believe and act on the assumption that other humans are conscious based on evidence that these skeptics claim to reject. We can see this more clearly if we consider the criteria for sentience: observable behavior, relevant physiology and evolutionary logic. Not only humans, but many nonhuman animals satisfy these criteria.

One of the basic considerations for choosing one scientific theory over another is parsimony, i.e., simplicity in explanation.4 That one explanation is far more complex than another to account for why something happens does not make it false. But it is, in principle, a reason to prefer the simpler alternative. Thus, parsimony leads us to consider that a large number of nonhuman animals are sentient beings. To accept the skeptical view presented above we would have to assume a lot of things that seem very unlikely. For example, we would have to assume that the same complex structures (such as areas of the brain responsible for pain processing in humans and also present in animals) and physiological processes (nervous system activity) have different purposes in animals than they have in humans. When an animal with an injured leg cries out, stops putting pressure on the leg, experiences rapid heartbeat, and produces opioid substances (associated in humans with relieving pain) and what we commonly call stress hormones, we would have to assume that all those things we are observing are present in an injured animal for completely different reasons than when they are present in an injured human.

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